1. Field of the Invention
The invention relates to a metallic porous product used as a reinforcement for producing a light alloy composite product having an aluminum alloy or a magnesium alloy as its structural metal and a method of producing the metallic porous product, and, more particularly, to a metallic porous product which is improved in wear resistance without causing aggravation of toughness, one of essential physical properties inherent by nature in metals, and a light alloy composite product made by the use of the metallic porous product with an effect of improving its physical properties, and a method of producing the metallic porous product and the light alloy composite product.
2. Description of the Related Art
Pistons and its associated parts of a diesel engine are typically made of a high silicon aluminum alloy, such as AC8A specified by Japanese Industrial Standard, which has low thermal expansion and high wear resistance. For example, a piston is subjected to tremendous thrust force repeatedly imposed on piston rings fitted in ring grooves of the piston on the firing strokes and a zone of the high silicon aluminum alloy piston where the ring grooves are formed is not always adequate in respect to wear resistance and fatigue deformation resistance. For this reason, further improvement of physical properties of the piston has been long desired.
The inventors of this application have studied improvement of light alloy composite products and metallic porous products preparatorily provided for producing the light alloy composite product. As a result of the study, as described in Japanese Patent Publications Nos. 2 - 30790 and 3 - 30708, the inventors have come up with a useful technique for improving physical properties of a light alloy composite product by impregnating a metallic porous product as a reinforcement of a light alloy composite product with a molten light alloy so as thereby to produce an intermetallic compound at an interface between these metallic porous product and impregnated light alloy. As described in Japanese Patent Publication No. 1 - 15347, the inventors have also come up with a light alloy composite product provided by filling metal powders, ceramic powders or carbon powders in open pores of a metallic porous product and impregnating it with a molten light alloy. These metallic porous products have a high porosity and are widely used as, for example, a catalyst support and a battery base because of a high filling factor for a catalyst or an active material.
There have been known as one of methods of producing metallic porous members having a porosity greater than approximately 90% a metal plating method such as described in, for example, Japanese Unexamined Patent Publication No. 57 - 174484 in which a metal is deposited on a foamed resin and a slurry coating method such as described in, for example, Japanese Unexamined Patent Publication No. 5 - 339605 in which a metallic porous product is made by sintering a metallic skeleton structure prepared by burning a foamed resin sheet impregnated with a metal powder slurry.
The prior art light alloy composite products still have some problems remaining unsolved. For example, even the light alloy composite products described in Japanese Unexamined Patent Publications Nos. 2 - 30790 and 3 - 30708 mentioned above have hardness between approximately 150 and 700 micro-vickers. In some applications of the light alloy composite product, desired wear resistance, which depends upon the hardness of a metallic porous product or the hardness of an intermetallic compound formed at an interface between the metallic porous product and base metal of the light alloy, is not always provided by that hardness of the light alloy composite product. In particular, in the case where the light alloy composite product is used as a material for part of a piston in which piston ring grooves are formed, it admits of improvement of physical properties. While the wear resistance may be improved by means of an increase in the volume portion of the metallic porous member relative to the light alloy composite product, this leads to an insufficient porosity with the result of increasing pressure necessary to impregnate a molten light alloy to approximately 30-300 kg/cm.sup.2.
While the prior art light alloy composite product described in Japanese Patent Publication No. 1 - 15347 mentioned above yields improved wear resistance due to powders of metals, ceramics or carbon powders filled in open pores of the metallic porous product, however, the powders tend to aggregate when filled in the open pores and consequently the problem that pressure necessary to impregnate a molten light alloy must be increased is left remaining unsolved.
In view of the above problems, in order for the light alloy composite product to yield significantly improved wear resistance, it has been long desired to disperse ceramic powders almost uniformly in the metallic porous product and to reduce the pressure necessary to impregnate a molten light alloy as low as possible.